The mechanical behaviors of interface between clay and concrete were investigated by unidirectional direct shear tests under different interfacial water pressure and normal stress conditions, from which the shear strength and stress-strain relations of interface were analyzed. After that, a constitutive model for the interface between clay and concrete was proposed based on the hyperbolic model considering the influence of interfacial water pressure. According to the test, it can be found that there is a hyperbolic relationship between shear stress and shear displacement, and the vertical displacement mainly expresses the shear shrinkage. The shear strength of interface is significantly correlated with the saturation state of soil near the interface. The shear strength of interface decreases obviously with the increase of interfacial water pressure when the surrounding soil changes from unsaturated state into saturated state. However, when the surrounding soil is saturated, the reduction of shear strength with the increase of interfacial water pressure becomes smaller, and the relationship between the shear strength and the effective normal stress is linear. The predicted result by the model corresponds with the experimental data, which illustrates that the proposed model is reasonable and practical.